A micromachine is also called MEMS (Micro Electro Mechanical System) or MST (Micro System Technology) and refers to a comprehensive system combining a minute mechanical microstructure and an electric circuit. The above-described microstructure is different from a general semiconductor element and has a three-dimensional structure, and a part of the microstructure is movable in many cases. The micromachine can have various functions of a passive element such as a sensor, an actuator, an inductor, or a variable capacitor, a switch, or the like. The electric circuit is generally formed using a semiconductor element and can control the operation of the microstructure, or receive and process a weak signal output from the microstructure.
Further, micromachines can be classified into groups according to their manufacturing methods. One is bulk micromachines in which microstructures are manufactured using crystal anisotropy of a silicon substrate, and the other is surface micromachines in which thin films are formed over various substrates to manufacture three-dimensional microstructures (Reference 1: Japanese Patent No. 3590283). In particular, the surface micromachines have been actively researched, especially in the United States, because a microstructure and an electric circuit can be formed over one substrate.